1. Field of the Invention
The present invention relates generally to orthopedic prosthetic devices such as implants and artificial joints and, more particularly, to an improved retaining mechanism for the tibial component of a knee prosthesis.
2. Description of the Prior Art
The knee joint consists of the bone interface between the distal end of the femur and the proximal end of the tibia. The tibial-femoral interface is covered by the patella, a sesamoid bone within the tendon of the quadriceps on the front of the thigh. This tendon attaches to the tibial tuberosity and the posterior surface of the patella, and glides over the femur. The femur has a medial condyle and a lateral condyle which are substantially smooth and articulate with the medial condyle and lateral condyle of the tibia. The tibial condyles are slightly cupped to receive the condyles of the femur.
When the knee joint is damaged, the natural bones may be unable to articulate properly. In some cases, a prosthetic replacement of the damaged section is required to restore normal use of the joint and reduce pain. Typically the entire joint is replaced by a surgical procedure which removes the ends of the corresponding damaged bones and replaces these ends with prosthetic implants.
A total knee prosthesis includes patellar, femoral and tibial components which are intended to replace the mating surfaces of the patient's femur and tibia. The tibial component very often consists of a metal tray which is anchored in the patient's tibia and a plastic insert made of a lubricous plastic material such as ultra high molecular weight polyethylene. The plastic insert is subject to wear and, therefore, may need to be replaced with time. Replacement of the insert is simplified if it is not necessary to remove the part (tray) which is implanted in the patient's tibia. Over the years, various retaining mechanisms have been developed for holding or locking the tibial insert to the tibial tray.
U.S. Pat. No. 4,298,992 discloses a well known total knee prosthesis known as the Insall/Burstein (I-B II) posterior stabilized knee. The I-B II knee has been in use for over nine years, and in thousand of clinical cases there has been no reported dissociation of the tibial insert and tibial tray. In the commercial product, the insert is laterally loaded into the tray and captured by anterior and posterior rails, each with a dovetail, which hold the insert in place. A clip passes through the anterior rail and is held in the tray by two tines that fit in a mating recess in the tray, below the lower surface of the insert. The clip also has a protruberence that fits in a recess in the plastic to prevent medial-lateral motion of the insert relative to the tray.
Unfortunately, this retaining mechanism cannot be used in a total knee replacement where the posterior cruciate ligament (PCL) is retained because insertion of the insert would interfere with the intact posterior cruciate ligament. Current retaining mechanisms for posterior cruciate ligament retaining total knee replacements are prone to dissociation of the tibial insert because the applied loads are partially resisted by the retaining mechanism which therefore is subject to failure.
An object of the invention is to provide an improved retaining mechanism for the tibial tray of a total knee replacement, including posterior stabilized and constrained condylar knees.
Another object of the invention is to provide a retaining mechanism for the tray of the tibial component of a knee prosthesis specially adapted for use with knee replacements in which the posterior cruciate ligament is to be retained.